Fluidized bed combustion (FBC) is a combustion technology used in power plants primarily to burn solid fuels. FBC plants are more flexible than conventional plants in that they can be fired on coal, coal waste or biomass, among other fuels. The term FBC covers a range of fluidized bed processes which include Circulating Fluidized Bed (CFB) boilers, Bubbling Fluidized Bed (BFB) boilers and other variants. Fluidized beds suspend solid fuels on upward-blowing jets of air during the combustion process, resulting in a turbulent mixing of gas and solids. The tumbling action, much like a bubbling fluid, provides a means for more effective chemical reactions and heat transfer.
During the combustion of fuels that have a sulfur containing constitutent, coal for example, sulfur is oxidized to form primarily gaseous SO2. In particular, FBC reduces the amount of sulfur emitted in the form of SO2 by a desulfurization process. A suitable sorbent, such as limestone containing CaCO3, for example, is used to absorb SO2 from the flue gas during combustion. In order to promote both combustion of the fuel and the capture of sulfur, FBC combustion operates at temperatures lower than conventional combustion systems. FBC systems operate in a range typically between about 780° C. and about 1000° C. Since this allows coal to combust at cooler temperatures, NOx production during combustion is lower than other coal combustion processes. Fluidized-bed boilers evolved from efforts to find a combustion process able to control pollutant emissions without external emission controls (such as scrubbers).
CFB boiler systems are generally associated with limestone feed systems for sulfur capture. Processed limestone fed to a boiler is typically conditioned by means of size reduction machines to specific size ranges to allow for the desulfurization process to proceed efficiently. If the particles are too large, the desulfurization process will not be efficient because there is insufficient limestone particle surface area to react with the flue gas. On the other hand, if the particles are too small, the limestone will be carried out of the desulfurization zone with the flue gas before it can react to remove the sulfur. Typically, limestone is fed to the boiler with a median particle diameter in the range of (as an example, but not limited to) about 100 to about 400 microns. In order to achieve this particle size range, unprocessed, raw limestone is reduced in both size and moisture content by size reducing machines. Presently, there are various machines available for crushing limestone, including for example, hammer mills, roll crushers and roller mills. Regardless of the type of equipment used for limestone crushing, the particles are dried either before or during crushing in order to produce a freely flowing material.
Traditionally, limestone is prepared separately from the boiler system, either on-site or by the limestone supplier. Prepared limestone is conveyed to a storage system in the boiler house from which it is thereafter metered and injected into the boiler. Experience has shown that the cost of prepared limestone using separate on-site systems or supplied from off-site vendors is expensive. In the case of separate, on-site systems a separate building and auxiliary fuel is used to dry the limestone. On the other hand, a limestone preparation and feed system may also be integrated with the boiler system itself, resulting in a significant reduction in capital and operating costs. Specifically, CFB boilers may be equipped with an integrated limestone preparation and feed system that resides in the boiler building. Such a system that dries and prepares limestone as needed is also referred to a Just-In-Time (JIT) limestone system.
The air system in a CFB is designed to perform many functions. For example, CFB air is used to fluidize the bed solids consisting of fuel, fuel ash and sorbent, and sufficiently mix the bed solids with air to promote combustion, heat transfer and control (reduction) of emissions (e.g., SO2, CO, NOx and N2O). In order to accomplish these functions, the air system is configured to inject air at various locations at specific velocities and quantities. Furthermore, an air system designed to maximize control (reduction) of one emission (e.g., NOx) may minimize control (hinder reduction) of another emission (e.g., SO2). Accordingly, the air system for CFB boilers is generally designed with the following distribution: Primary Air (PA) accounts for approximately 50% of the total system air (more generally in a range of about 35% to about 60% of the system air); Secondary Air (SA) accounts for approximately 35% of the total system air (more generally in a range of about 30% to about 45% of the system air); and Tertiary Air (TA) accounts for approximately 15% of the total system air (more generally in a range of about 5% to about 20% of the system air).
Primary air is injected through a grate at the bottom of the furnace, while secondary air is injected through ports mounted in the furnace walls (e.g., front, rear and side) above the furnace grate. Typically, secondary air is divided into at least two vertical injection planes above the furnace grate. It is also typical to evenly split the air to each plane. Thus, for example, if SA represents 40% of the total combustion air, a typical split would be 20% in the lower SA plane and 20% in the upper SA plane. Tertiary Air is air used to fluidize external heat exchangers, cyclone siphon seals (seal pots) and other, auxiliary equipment. This air enters the furnace through dedicated openings in the furnace walls.
JIT limestone systems typically employ a roller mill (i.e., an air swept crusher) to crush the limestone prior to feeding into a CFB boiler, utilizing a significant portion (e.g., about 20% to 30%) of the combustion air to entrain and convey crushed limestone from the mill to the CFB furnace. This portion of the combustion air (also referred to as secondary air) is typically fed into the furnace near (above) the primary air distribution grate. Given the high percentage of total secondary air, the JIT air must be split between the lower and upper SA planes. The conditioned sorbent, entrained in the JIT air, is therefore injected at both SA planes. However, given that a significant amount of secondary combustion air is utilized for entraining and conveying sorbent particles to a lower portion of the furnace in proximity to the primary air distribution grate, the ability to control SO2 emissions in a JIT limestone system is still somewhat limited.